Fluid sensing device using discotic liquid crystals

ABSTRACT

The present invention discloses a fluid sensing device comprising a substrate having at least one type of dicotic liquid crystals disposed therein and arranged to form an array of columnar structures and a contact means for measuring the flow of electric charge through the upper part of the columnar structure surface. Also disclosed is a method for detecting a fluid comprising the steps of: exposing a sensing device to a fluid so that the fluid interacts with the surface of the discotic liquid crystal; applying a voltage to the contact means; measuring a flow of electric charge; and, analyzing a variable current flow to identify the fluid.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to sensing devices, and aspects thereof, whichexploit the unique properties of discotic liquid crystals (DLC).

2. Description of the Related Art

DLCs essentially comprise an aromatic core surrounded by severalaliphatic side chains. Typically, a number of aromatic cores arepositioned in an aligned, stacked fashion so as to provide for acolumnar arrangement. Further, the columns tend to organise into a twodimensional superlattice providing, for example, a hexagonal structure,hexa-alkoxytriphenylenes are well known representatives of thisstructure of DLCs.

DLCs attracted considerable attention when it was discovered that thecolumnar phase structure of DLCs was suitable for fast transport ofcharge carriers. More specifically, it was established that theorientation of the columns directed the flow of this charge because thecharge essentially travelled along each column and was further insulatedfrom adjacent columns by the aliphatic side chains attached to thearomatic cores. As a result of this knowledge the use of DLCs in theelectronics industry has grown and it is of note that it is the transferof electric charge along the axis of the columnar DLCs that has beenexploited.

However we disclose in this Patent Application a new property of DLCsand a novel way in which this new property can be exploited.

Fluid sensing devices, and in particular gas sensors, are becomingincreasingly important for monitoring industrial environments. Inparticular, they are desirable for use in the chemical industry wherethe detection of leaks and in particular the detection of leaks of ahazardous nature must be continually monitored. As a result of thisvarious gas sensors have been developed. The most sophisticated is basedon polymer technology and essentially involves the interaction of agaseous molecule with a given polymer and the recording of a response asa result thereof. More specifically, the sensor uses electricallyconducting organic polymers based on heterocyclic molecules such aspyrrole. Each polymer is a different functional unit that displaysreversible changes in conductivity when it is exposed to polar volatilechemicals. Usually, an array of polymers are provided and theinteraction of a gas molecule, or a cocktail of gas molecules, whenexposed to each of said polymers in said array is monitored and thesubsequent response, or fingerprint, is recorded. Thereafter when thesame sensor array is exposed to the same gas, or combination of gases,the same response, or fingerprint, is noted. In this way gas sensors canbe “trained” to detect different gases, or combinations of gases, and sobe programmed to monitor different, environments for leaks.

However, the aforementioned sophisticated technology is not sufficientlysensitive to detect all kinds of gases and in particular it is not ableto detect all organics. Notably, it cannot detect non-polar hydrophobicorganics such as benzene and as a result of this its application is notuniversal.

BRIEF SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a fluid sensor andin particular a gas sensor which detects a wide range of gases and inparticular organic based gases.

It is yet a further object of the invention to provide a sensor and inparticular a gas sensor based on the use of DLCs.

It is yet a further object of tie invention to provide a sensor arraycomprising at least one and preferably a plurality of DLCs and ideally aplurality of differing DLCs.

It is yet a further object of the invention to exploit a new use ofDLCs, that being for the detection of fluids and in particular gases andmore particularly further still non-polar organic based gases.

It is yet a further object of the invention to provide a sensor thatoperates in real time.

According to a first aspect of the invention there is therefore provideda fluid sensing device comprising a substrate on which there is providedat least one type of discotic liquid crystal and further wherein thereis also provided contact means adapted so as to measure the flow ofelectric charge through the upper part of the said discotic liquidcrystal.

It will therefore be apparent from the above, to those skilled in theart, that we have identified a novel property of DLCs, that being theability to conduct a surface charge, that is a charge generallyperpendicular to the axis of the columns forming the DLCs. Moreover, wehave also discovered that this surface conductivity can be affected byfluids and in particular gases such as organic based gases. It thereforefollows that the surface conductivity can be used to monitor the levelsof, or existence of, fluids and in particular gases in a givenenvironment.

Advantageously the effect on surface conductivity is very fast and sothe device is able to operate in real time.

In a preferred embodiment of the invention a plurality of discoticliquid crystals are provided and ideally on a single substrate.Preferably the plurality of discotic liquid crystals are positioned onsuch substrate so as to provide for an array.

In the preferred embodiment of the invention the response of differentdiscotic liquid crystals to different fluids or gases can be determined,and where an array is provided a given gaseous molecule, or combinationof gaseous molecules, can interact with the said array so as to providea given, typically unique, response. This response can then be recordedand used for future analysis of gases, either identical to the originalgas, or gases, or differing therefrom.

Ideally, the said device is also provided with an information storageand retrieval facility whereby data relating to different fluids and inparticular gases can be stored and accessed so that analysis of gases orenvironments can be facilitated.

In yet a further preferred embodiment of the invention said at least onediscotic liquid crystal is 2, 3, 6, 7, 10, 11 hexa-hexyloxytriphenylene(HAT6).

More preferably still said discotic liquid crystal comprises at leastone such crystal shown in table 1 and exemplified in FIGS. 12 and 13.Ideally said discotic liquid crystal comprises a plurality of thediscotic liquid crystal shown in table 1 and exemplified in FIGS. 12 and13, and ideally each gas sensing device comprises a selected combinationof said discotic liquid crystals which combination is selected havingregard to the purpose of the sensor. Therefore, for example, where givendiscotic liquid crystals are shown to be particularly sensitive to agiven gas, or combination of gases, then these discotic liquid crystalswill be employed in sensors used to detect gases, or combinations ofgases, for which they have exemplified favourable sensitivity.

Although the invention has been described with reference to the discoticliquid crystals shown in table 1 and exemplified in FIGS. 12 and 13 itwill be understood by those skilled in the art that the invention is notto be limited by the examples of discotic liquid crystals specified inthis application, rather the invention lies in the realisation thatdiscotic liquid crystals can be used, because of their surfaceconductivity, to detect fluids and in particular gases. Thus the numberand nature of discotic liquid crystals that can be used in the inventionare limitless, as is their combination, selective or otherwise,typically for use in an array.

Surprisingly, we have found that our device responds to different gaseswhatever the thickness of the DLC layer and we consider this to bebecause the upper conducting surface remains constant. Indeed, we havefound that the thinner the layer the lower the surface resistance and sothe greater the conductivity. As a result of this we prefer to usesensors that comprise a relatively thin film of at least one DLC, forexample, the said film is typically less than one micrometre and morepreferably still less than 0.5 of a micrometer and ideally in the orderof 0.1 micrometers.

According to a second aspect of the invention there is provided a sensorarray, for use in a fluid sensor, comprising at least one type ofdiscotic liquid crystal.

In a preferred embodiment of the second aspect of the invention aplurality of different discotic liquid crystals are used. Morepreferably still means are provided to measure surface current flowacross each type of discotic liquid crystal.

According to a yet third aspect of the invention there is provided theuse of at least one discotic liquid crystal for use in detecting a fluidand in particular a gas, especially an organic based gas such as anon-polar gas.

Whilst we would not intend for the invention to be limited by thefollowing explanation, we consider that discotic liquid crystalsfunction as fluid, and in particular gas, sensors because of the abilityof the aliphatic side chains to interact with said gases and so affectthe organisation at the surface of the discotic liquid crystalstructure. The enhancement or depression of surface organisation thusaffects the ability of the surface to conduct charge and we speculate,that in this way, a given fluid interacts with the surface of a discoticliquid crystal to affect surface organisation and thus charge flowtherethrough.

We believe that DLCs will be especially useful for sensing non-polarhydrophobic volatile chemicals in vapour sensing instruments usuallyreferred to as “the electronic nose”. These instruments, which detectsmell levels down to parts per billion normally use an array of 8, 16 or32 separate gas sensors. It is thought that DLCs may be used in suchsensors and exploited because of the ability of DLCs to detect non-polarhydrophobic gases.

An embodiment of the invention will now be described by way of exampleonly with reference to the following Figures wherein:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the chemical structure of a selected discotic liquidcrystal;

FIG. 2 is a diagrammatic representation of the organisational structureof a discotic liquid crystal;

FIG. 3 is a diagrammatic representation of a discotic liquid crystalfilm on a substrate;

FIG. 4 is a diagrammatic illustration of a fluid sensor in accordancewith the invention;

FIG. 5 is a graph showing a measure of resistance, and so conductivity,of a selected discotic liquid crystal surface, in response to a varietyof organics;

FIG. 6 is a graph showing a measure of resistance, and so conductivityof a selected discotic liquid crystal surface in response to aromaticand aliphatic hydrocarbons;

FIG. 7 is a graph showing a measure of resistance, and so conductivityof a selected discotic liquid crystal surface, in response to aliphatichydrocarbons of varying chain lengths;

FIG. 8 is a graph showing a measure of resistance, and so conductivityof a selected discotic liquid crystal surface, in response to variousesters;

FIG. 9 is a graph showing a measure of resistance, and so conductivityof a selected discotic liquid crystal surface in response to variousketones.

FIG. 10 is a graph showing how the increasing length of carbon chainsaffects the resistance, and so conductivity, of the surface of aselected discotic liquid crystal having compensated for the partialpressure of the gas;

FIG. 11 is a graph showing a measure of resistance, and so conductivityof a selected discotic liquid crystal surface, in response to discoticliquid crystal film thickness;

Table 1 is a list of discotic liquid crystals suitable for working theinvention;

FIGS. 12 and 13 show the chemical structure of the discotic liquidcrystals referred to in table 1.

FIG. 14 is a list of discotic liquid crystals suitable for working theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures and firstly to FIG. 1, there is shown thechemical structure of a selected discotic liquid crystal HAT6. It can beseen that the structure comprises an aromatic core surrounded by anumber of aliphatic side chains. HAT6 exists in a crystalline phasebelow 67° C. and is transformed into a liquid phase at temperatures inthe order of 100° C.

The basis structure shown in FIG. 1 is typical of all discotic liquidcrystals in that they essentially comprise an aromatic core surroundedby a number of aliphatic side chains. Moreover, the liquid crystallinestate is located between a crystalline and isotropic liquid state.

In FIG. 2 there is shown the organisational arrangement of a film ofdiscotic liquid crystals. Essentially, the aromatic cores are alignedand stacked so as to form columnar arrangements, thus the aliphatic sidechains are provided between the aromatic columns and so act as spacers.

Furthermore, they also act as insulators so ensuring that, inconventional applications, electric charge transfer is directed alongthe longitudinal axis of the columns.

Discotic liquid crystals are known to have a number of favourableproperties, one being their wetting property and thus the ability toensure efficient molecular contact with electrodes and the other beingthe high degree of internal order which occurs with the semi-fluidity ofdiscotic liquid crystals. The former property is advantageous in termsof construction of devices using discotic liquid crystals and the latterproperty is advantageous in that it provides for “self-healing” if adisturbance should occur at the molecular level.

FIG. 3 shows the arrangement of discotic liquid crystals on a substrate.It can be seen that the organisation of the columnar structure tends tobreak down towards the surface and indeed it is this property which weexploit in this invention. However, we have found that despite thethickness of the discotic liquid crystal film the surface conductivityremains and therefore we conclude that whatever the thickness of thefilm the advantageous surface structure remains. Indeed, data to bepresented hereinafter will show that thin films of discotic liquidcrystals are preferred.

In FIG. 4 we show a sensing device in accordance with the invention. Thedevice essentially comprises a substrate 1 on which there has beendeposited at least one discotic liquid crystal 2 existing in theliquid-crystalline state and so displaying the organised structuretypical of its type.

Positioned at selected locations, that is on either side of the discoticliquid crystal's surface, and so in opposing fashion, are a pair ofcontact means 3 such as electrodes. As aforementioned, the wettingproperties of the discotic liquid crystal ensures that once the DLCsfilm is applied to the substrate and then placed in contact with thecontact means good electrical contact is provided. Ideally the contactmeans comprise a pair of electrodes to which a voltage can be applied soensuring that current can flow.

As previously hypothesised exposure of the device, and in particulardiscotic liquid crystals to a fluid and ideally a gas results ininteraction of the gas with the discotic liquid crystal and inparticular the surface of the discotic liquid crystal so as to affectthe surface conductivity of the discotic liquid crystal. Various gasesreact differently with the discotic liquid crystal in order to providefor variable current flows. In this way each gas provides a variablecurrent which can be used, either in isolation or in combination withother such readings using different discotic liquid crystals, toidentify it.

In copending unpublished GB patent application number 9608774.7 isdescribed for example a method for analysing the unique signatureobtained in the form of a response as a function of frequency.

The sensing device may therefore be of such diverse applications as:process control, by monitoring organic chemical reactions in real time;environmental monitoring, by early detection of contamination; hazarddetection, by rapid indication of the presence of hazard gases and thelike.

Test Data

In the following tests the device shown in FIG. 4 was employed using thediscotic liquid crystal shown in FIG. 1. However, it is within the scopeof the invention to employ the use of any one or more of the discoticliquid crystals shown in table 1, and corresponding FIGS. 12 and 13, andindeed the selection of a single, or combination of, discotic liquidcrystal (s) will be discussed hereinafter.

In addition to the discotic liquid crystals specifically describedherein it is also envisaged that discotic liquid crystals havingmodified aliphatic side chains may also be used. In particular, discoticliquid crystals having side chains which are engineered so as to berelatively, hydrophobic, hydrophilic, long in length, short in length,high in dipole moment, low in dipole moment or otherwise may be used.

Referring therefore to FIG. 5 it can be seen that the sensor of theinvention was able to distinguish between acetates such as Ethyl Acetateand ring structures such as Benzene and Toluene.

FIG. 6 shows that the sensor of the invention was also able todistinguish between aromatics and aliphatics and moreover that thisability to distinguish is polarised such that a ring structure such asBenzene or CycloHexane provides for a reduction in resistance, and so anincrease in current flow, whereas an aliphatic such as n-Hexane providesfor an increase in resistance and so a reduction in current flow.

In FIG. 7 it can be seen that the device of the invention can be used todistinguish between different aliphatics and in particular the chainlength of different alipliatics. Summarily, the greater the carbon chainlength the lower the resistance and so the greater the conductivity.This is further also shown in FIG. 10 where it can be seen that there isa linear relationship between resistance at the discotic liquid crystalsurface and chain length.

In FIGS. 8 and 9 it can be seen that the device of the invention candistinguish between different esters and ketones, respectively, and sohas a wide range of application.

It is thought that prior art devices are able to detect those organicsshown in FIG. 9 but are unable to detect those organics shown in FIG. 8.It is therefore apparent that this invention is able to detect bothtypes of gases and therefore can be used to detect either non-polargases i.e. those shown in FIG. 8 which are currently undetectable usingprior art devices, or a mixture of polar and non-polar gases.

All of the aforementioned results illustrate the sensitivity of discoticliquid crystals in detecting different gases and in particular organicbased gases.

In a preferred embodiment of the invention, not shown, we prefer to usea device comprising an array of selected discotic liquid crystalswherein the surface current flow for each of said selected discoticliquid crystal in said array can be measured. Typically, the discoticliquid crystals selected for use in the array are chosen on the basis oftheir sensitivity to particular fluids. In this way devices can becustomised according to a users requirements. Alternatively, where thepurpose of the device is unknown a plurality of discotic liquid crystalscan be selected on the basis of the range of fluids that can bedetected. Ideally any one or more of the discotic liquid crystals shownin table 1 and exemplified in FIGS. 12 and 13 are used. However, it isnot intended that this invention should be limited to the specificdiscotic liquid crystals specified herein, rather the invention mayemploy any one or more known discotic liquid crystal(s).

In working the invention we have discovered that the conductivity at thesurface, and across the surface of the discotic liquid crystalarrangement, remains despite the thickness of the discotic liquidcrystal film. Indeed, we have surprisingly found that as the filmthickness decreases the surface resistance decreases and so the surfaceconductivity increases, see FIG. 11. As a result of this we prefer touse devices which include a thin film of at least one discotic liquidcrystal. Ideally we prefer devices which include at least one film thatis less than one micrometre. Ideally further still we prefer to usefilms that are less than 0.5 micrometers and ideally in the order of 0.1micrometer.

The provision of such a thin film is effected using the followingmethod.

A solution of known composition is made up by dissolving the requiredweight of discotic liquid crystals in a low boiling point solvent (suchas diethylether or carbon disulphide). Thin films are then formed on theelectrode surface by casting a known volume of solution followed byevaporation of solvent. Homeotropic alignment of the thin film isachieved by suitable heat treatment which consists of heating the filmto a temperature above its clearing point into the isotropic phase,followed by a slow cooling into the liquid crystalline phase.

Thus we describe herein the use of discotic liquid crystals to detectfluids and in particular gases by measuring the surface discotic liquidcharge associated with the interaction of said fluid and said discoticliquid crystal.

What is the claimed is:
 1. A fluid sensing device comprising a substrateon which at least one type of a discotic liquid crystal (DLC) comprisinga film having a thickness of less than one micrometer is arranged toform an array of columnar structures, said array of columnar structureshaving an upper part adapted for interaction with a fluid to be sensed,contact means comprising a plurality electrodes which are positioned onopposing sides of said array of columnar structures so that an electriccharge applied to at least one electrode flows generally across asurface of the upper part of said array of columnar structures in adirection substantially perpendicular to a vertical axis of saidcolumnar structures, wherein said fluid sensing device measures a changein the flow of electric charge caused by a change in surfaceconductivity across the surface of the upper part of said array ofcolumnar structures, said change representing the interaction of saidfluid to be sensed with the surface of the upper part of said array ofcolumnar structures.
 2. A fluid sensing device according to claim 1,which provides fluid sensing in real time according to a change in theflow of electric charge measured by said contact means across thesurface of the upper part of said array of columnar structures.
 3. Afluid sensing device according to claim 1, further comprising means forinformation retrieval of data relating to the interaction of variousfluids and gases with said at least one type of discotic liquid crystal.4. A fluid sensing device according to claim 3, wherein the retrieveddata includes information regarding the interactions of combinations ofsaid various fluids and gases with said at least one type of liquidcrystal.
 5. A fluid sensing device according to claim 1, wherein said atleast one discotic liquid crystal is selected from the group consistingof 1-6 β-triphenylenes and tricycloquinazolines, 1-6 α-triphenylenes andtricycloquinazolines, and substituted phthalocyanines.
 6. A fluidsensing device according to claim 5, which comprises at least onediscotic liquid crystal is selected from the group consisting of2,3,6,7,10,11-Hexa(hexyloxy)triphenylene (HAT 6); 1-Nitro2,3,6,7,10,11-Hexa(hexyloxy)triphenylene (HAT6-NO₂);2,3,7,8,12,13-Hexa-(2-(2′-MethoxyEthoxy)-Ethoxyl)TriCylcoQuinazoline(HMEETCQ); and 1,4,8,11,15,18,22,25-Octa-(Octyl)Phthalocyanine (PC8). 7.A fluid sensing device according to claim 1, wherein said at least onediscotic liquid crystal has side chains which are selected from thegroup consisting of hydrophobic, hydrophilic, high in dipole movement,and low in dipole movement.
 8. A fluid sensing device according to claim1, wherein said at least one type of discotic liquid crystal comprises acombination of discotic liquid crystals selected on the basis of atleast one of sensitivity to particular fluids to be sensed by said fluidsensing device, and sensitivity to a range of particular fluids to besensed by said fluid sensing device.
 9. A fluid sensing device accordingto claim 8, wherein at least one type of discotic liquid crystal of saidcombination of discotic liquid crystals comprises 2,3,6,7,10,11,hexa(hexyloxy) triphenylene (HAT6).
 10. A fluid sensing device accordingto claim 1, wherein said film of said at least one type of discoticliquid crystal has a thickness of less than 0.5 micrometers.
 11. A fluidsensing device according to claim 10, wherein said film has a thicknessless than 0.1 micrometers.
 12. A method for detecting a fluid comprisingthe steps of: exposing a sensing device as defined in claim 1 to a fluidso that said fluid interacts with the surface of at least one discoticliquid crystal, applying a voltage to the contact means, measuring aflow of electric charge between the contact means, and analyzing avariation in flow of electric charge to identify the fluid.
 13. A methodaccording to claim 12, wherein said analyzing step includes comparingthe variation in flow of electric charge for a particular discoticliquid crystal with values in an information storage means.
 14. A sensorarray for a fluid sensing device, comprising a plurality of discoticliquid crystals having a thickness of less than one micrometer on asubstrate, and means for measuring a surface current flow across asurface of said discotic liquid crystals.
 15. A method for thepreparation of a fluid sensing device comprising the steps of: (a)providing a film of at least one type of discotic liquid crystal havinga thickness of less than one micrometer on a substrate, (b) providingcontact means adapted for measuring a flow of electric charge across anupper part of the film of said at least one type of discotic liquidcrystal.
 16. A method according to claim 15, wherein step (a) includesaligning the thin film planarly on the substrate. contact means.
 17. Amethod according to claim 16, wherein said substrate comprises anelectrode surface in contact with said contact means.
 18. A methodaccording to claim 15, wherein said substrate comprises an electrodesurface in contact with said contact means.